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New submitter EngnrFrmrlyKnownAsAC writes "Communicating with lasers has become the hot new thing. While most researchers are seeking faster throughput, NASA set its sights in a different direction: the moon. They recently announced the first successful one-way laser communication 'at planetary distances.' What did they send? An image of the Mona Lisa, of course. 'Precise timing was the key to transmitting the image. Sun and colleagues divided the Mona Lisa image into an array of 152 pixels by 200 pixels. Every pixel was converted into a shade of gray, represented by a number between zero and 4,095. Each pixel was transmitted by a laser pulse, with the pulse being fired in one of 4,096 possible time slots during a brief time window allotted for laser tracking. The complete image was transmitted at a data rate of about 300 bits per second.'"

Each pixel was transmitted by a laser pulse, with the pulse being fired in one of 4,096 possible time slots during a brief time window allotted for laser tracking.

I have an idea. What if, instead of this encoding, they used twelve time slots for each pixel and, by either sending or not sending a pulse, transmitted a small amount of information with each (non)pulse? Then, they could interpret the slots by repeatedly adding a one or zero and multiplying the whole thing by two. I think I've read about it somewhere...

What if, instead of this encoding, they used twelve time slots for each pixel and, by either sending or not sending a pulse, transmitted a small amount of information with each (non)pulse?

Doesn't work that simply or easily over an unreliable medium... A BIRD flying by at the wrong time could turn a string of ones into zeros. Plus there are similar issues of clock-sync... With a long run of zeros, is your timing precise enough on both ends to ensure that you know EXACTLY how many zeros there were supposed

If you can't send zeroes, you could encode data in the length of a laser pulse instead. Use error correction techniques to reduce the chance of errors. A checksum should have very low overhead but decrease your chances of errors immensely. If you're feeling silly, you could even use differently coloured lasers and hope they all hit near the same spot. Disco SMS:D

Because they can't reliably send individual bits. If you RTFA (I know, I know...) it shows that there is a fair bit of error and quite a few lost pixels. Rather than sending bits they send a pulse of a certain length per pixel, and if the edge of that pulse is distorted somehow they just lose some intensity resolution and don't end up with totally corrupted digital data.

It's kind of analogue. The timing method they use is a bit like PWM with one cycle per pixel, and actually there are far fewer than 4096 shades reliably transmissible, that is just the range they measure.

The laser-tracking protocol is defined to run at 25 pulses a second; pulling them back and forward by tiny amounts, to take advantage of the electronics in the orbiter that are designed to measure tiny time differences in order to do the LIDAR altimetry, is a really nifty classic NASA hack.

But the press release did not make a good job of pointing out that NASA were working under that restriction. Obviously if you were trying to do laser communication you'd do something else; ESA have done 50Mbit/second lase

He who would negotiateth a handshake of that breadth must answer me these questions three, Ere the ATDT ye see...
0. What is your FidoNet node address?
1. What number of in & out dials have you?
2. Whatis the land area coverage of an unladen local call?

1. What number of in & out dials have you?7 / 1 (8 chans of a frac pri)

2. What is the land area coverage of an unladen local call?About half of the area code, guessing 100 square miles?

I never understood why some parts of 614 were local but others were long distance, while at the same time a small part of 740 was local to me yet a different area code.I had to route mail to another board across town in 614, where he could reach the other half of 614 locally, just to avoid minutely charges.

My 8 PRI channels were to my home (well, to my parents home at the time) and mostly for dialin. I rocked Oblivion/X by the time I was on fido. One line floated for scheduled callouts, but none dedicated to that.Once I discovered the Internet in '89, first one then later two channels were dedicated to PPP.By '92 I was getting less than 5 calls a day to the board, and shortly converted my whole frac PRI to be dedicated Internet, and I pretty much gave up the sysop role for good in exchange for EFnet as things turned out. Even ran an efnet server for a short time back in '95 i think it was.

While I can say for certain that communications have only changed for the better as far as the Internet goes, there is still a lot I miss from those days, even though I wouldn't want to go back to that for anything.

150 cps isn't even a good typing speed. I've often outtyped 1200 bps modems and I can definitely outread them. I used to hang out on a five-liner with all 1200bps USRs in scruz called XBBS and the modems were agonizing.

lasers used for communication in space is very important/useful due to the distances involved. the more focused you can send, the less energy needed to compensate for signal spread...probably not the right terms, but in general terms that's the first thing I thought of. The distances involved in space take some real work to deal with...Everything gets harder to do when you talk about these kinds of distances....
You may think a trip down to the corner store is a long way to go, but that's just peanuts to sp

I believe the current theory on laser communication in space is that it could get up to some serious bandwidth at inter-planetary distances - think 100 mbit/s to 1 Gbit/s - but obviously it's not really been tried yet. Curiosity is only sustaining something like 1.5 mbit/s even even with help from the Mars orbiters.

I'm guessing the real benefit of this type of work was getting some actual data on the types of things which affect tracking and receiving lasers in space, even if only at very low bitrates.

I imagine that an environment that isn't all-male wouldn't be so pleased with the idea of "tradition."

If a person has a problem with the image of a woman from the shoulders up, I'm certain they would also have issues with the current image of a woman from the shoulders up.

Besides, I doubt anything you could do would please a person who is offended by a picture of a female face, no matter which face it happens to be. They would also likely be offended by just about anything else, including squiggly lines in a stock 60's TV test pattern.

There's really nothing you can do about such people, they will be offend

They have mirrors on the moon, that we routinely bounce lasers off of to measure distances and do Relativity experiments with. It's suddenly difficult to transmit information via laser? Why so slow? Why was this an accomplishment?

They have mirrors on the moon, that we routinely bounce lasers off of to measure distances and do Relativity experiments with. It's suddenly difficult to transmit information via laser? Why so slow? Why was this an accomplishment?

To demonstrate a line of sight transmission, from any possible point of orbit? Think about it. They are developing towards a true subspace solution.

If laser communication overtakes radio for our own space equipment, it might explain the Fermi paradox - we cannot detect alien civilizations because the communicate with lasers (emitting no radio signals at all), making them undetectable to those not in the path of the beam.

It doesn't matter. Laser. Radio. Gama ray.Doesn't matter. At these distance the systems are, no matter how well focused, diffraction limited. Just like we can't build a mircoscope to see infinitely deep into the smal we cannot build a laser com with perfect focus. Diffraction wins. We can cheat a little, but not over these distances.

We could see laser flashes just as easily as hear radio waves from parabolic dishes.

Why not use 2 different wavelength lasers (Or even 3 or more)For two lasers (let it be greeen and blue) it would be binary transmission,"4096" fits into 13 bits. Image transmission would be 315 times faster.

Use one laser. Actually there is no need for second wavelength for binary transmission.I assume that actual cause of "analog" data transfer is that there is some possibility thattheir data transfer is not very precize on receiving side. They could send for example0,0,10,10,4095 and receive 0,0,11,9,4094That still would be ok for "analog" image, but not for binary transmission.

As I've been saying for years SETI doesn't have a hope in h**l finding the aliens because they use the much more efficient point to point message casting as opposed to the broadcasting in every direction used here on earth. Why use the inefficient method sending your message/data/... everywhere when it is really only destined for 1 place.

I think in 100 years we'll look back and see that the use or radio and the inefficient broadcasting methods was a short segment in our history. It will likely be the same for other developing races.

I recall a few years back they actually found something that looked like real alien communication. It couldn't be captured again. Of course it coul'n't be found again. We were no longer behind the target of the message beam.

But aren't laser pulses that coincidentally point our way detectable? If you have gajillion satellites and spaceships all about, then every now and then one will line up with Earth.

Will the aliens' laser have spread out enough to cover a significant part (or all) of Earth by that point, or will it still be pretty small? It's no good having the laser light land in New Zealand if your receiving equipment is in Australia, after all...

Only if you're looking for them. Optical SETI has barely begun, and only with a fraction of the (miniscule) resources that radio SETI has. We at least *think* we know how to do radio search, but there are so many bizarre natural sources in the optical range that I don't think they're sure what to look for.

Why use the inefficient method sending your message/data/... everywhere when it is really only destined for 1 place.

I think you're forgetting about broadcast transmissions. You know, TV, trunked mobile communications...messages that will reach multiple locations simultaneously. Now the distances involved and the spreading loss, that's another issue. There is only so much coherent integration that can be done usually.

Fist it is not 4096 bits. It takes 12 bits to make up 4096 possibilities. Also, don't you think that the scientists factored the 12 bits into their 300bps calculation? If there were only 300/12= 25 slots per second the data rate would be 300 bps.

So, the moon, which is 409,073 kilometers away at its furthest is "planetary distances"? What does that make the distance to Venus, which is 41 million kilometers at its closest or Mars, which is 56 million kilometers at its closest. Seems to me that this is only over about 1% of the shortest distance you could actually consider "planetary distances".

They way they're doing it is too damned easy. I'd throw a little challenge into it by requiring that low bits must transmitted by bouncing them off the Apollo laser reflectors. Might require spinning up LRO to about 3000 rps unless it has two sensors.